Type 1 or Insulin-Dependent Diabetes Mellitus (IDDM) arises in genetically predisposed individuals as a consequence of immune mediated destruction of the pancreatic islet insulin secreting beta-cells. The onset of clinical symptoms of diabetes represents the end point of a chronic progressive decline in beta-cell function, and it appears only when the majority of beta-cells have been lost. Since IDDM develops insidiously, often years after the induction of the pathogenic immune-mediated destructive process, it can be predicted using immunological markers and tests of insulin secretion. The Diabetes Prevention Trial of Type 1 Diabetes (DPT-1) has been designed to test whether intervention during the prodromal period of the disease can delay its clinical onset.